1. Field of the Invention
The invention relates to a fixing unit of an image forming apparatus for heating a paper sheet. An image is formed on the paper sheet by supplying toner on an electrostatic latent image formed on a sensitive drum of a developing unit. The image is transferred with a heating roller to fix the toner image transferred to the paper sheet. Specifically, the invention relates to an inexpensive fixing unit which, even if a halogen lamp disposed in the heating roller is thermally expanded or contracted due to the supply of electricity to the halogen lamp, can respond flexibly to the thermal expansion or contraction without breaking the halogen lamp and supply the electricity reliably.
2. Description of Related Art
Various types of image forming apparatuses such as laser printers or similar devices that use electrophotography have been proposed. The specification and drawings of Japanese Published Patent Document No. 9-319201, for example, describes an image forming apparatus that generally includes a paper carrying unit, a laser scanning unit, a toner supply unit, a developing unit, a fixing unit and similar components. In such an image forming apparatus, an electrostatic latent image is formed on the surface of a sensitive drum in the developing unit according to image data. The electrostatic latent image is formed by the laser scanning unit. Toner is supplied to the sensitive drum according to the electrostatic latent image by the toner supply unit to form a toner image on the surface of the sensitive drum. The toner image is transferred to the paper sheet carried into the developing unit from the paper carrying unit by a transfer roller and the sensitive drum. The paper sheet subsequently is heated by the fixing unit to fix the toner image.
The fixing unit, in general, has a rotatable heating roller and a rotatable pressing roller which presses the heating roller. The toner image of the paper sheet is heated and fixed by the heating roller and the pressing roller. The heating roller is a rotatable roller. The rotatable roller is an aluminum cylinder, the surface of which is coated with Teflon resin. Moreover, a halogen lamp, which is used as a heating source, is fixed to the cover of the fixing unit and is disposed in the heating roller.
The halogen lamp includes a lamp, both ends of which are fixedly provided with electrodes coated with a coating material such as ceramic. The electrodes that are fixed at the both ends of the lamp are supplied with electricity by a harness connected thereto.
Typically, the halogen lamp that is used in the above-described fixing unit is expensive because the electrodes are previously fixed to both ends thereof and the electrodes are coated with a coating material such as ceramic. Therefore, providing an expensive halogen lamp as the heating source of the fixing unit increases the cost of the fixing unit and thus increases the total cost of the image forming apparatus.
Therefore, using a halogen lamp, whose both ends are not fixedly provided with electrodes and whose electrodes are not coated with a coating material, will reduce the cost of the fixing unit. In other words, using bare lead pins that extend from both ends of the halogen lamp will reduce costs.
However, when a halogen lamp, provided with bare lead pins extending from both ends thereof, is used as the heating source of the fixing unit, a problem arises regarding what structure is used for supplying electricity to the halogen lamp via each of the lead pins.
The halogen lamp is thermally expanded or contracted when it is turned on or off. The lead pins can be fixed with a large fixing force in contact with a conductive material that supplies electricity. However, if such a large fixing force is used, the pins are prevented from moving even if the halogen lamp expands or contracts due to temperature changes. As a result, the stress caused by the thermal expansion or contraction is applied to the joint portion of the lead pins and the lamp bulb to break the lamp bulb. However, if the fixing force between the lead pins and the conductive material is small, the lead pins move when the halogen lamp is thermally expanded or contracted. This movement prevents the lamp bulb from breaking. Electricity, however, may not be stably supplied to the halogen lamp via the conductive material, which prevents the toner image on the paper sheet from being reliably heated and fixed.
The inventor has made experiments and studies under these circumstances and has focused on a static friction force generated between the lead pins of the halogen lamp and the conductive material for supplying electricity. The invention is based on the static friction force which can flexibly move the lead pins when the halogen lamp is thermally expanded or contracted and can stably supply electricity to the lead pins.